Lignin is contained in many plants including trees and grass together with cellulose and polysaccharides other than cellulose (generically referred to as hemicellulose). For example, the amount of lignin contained in trees is about 20 to 30% by mass, although it varies depending on the type of a tree. Note that the amount of cellulose contained in trees is about 40 to 50% by mass, and most of the remaining components are hemicellulose.
In plants, these three components are present in a form associated with each other, which is generally called lignocellulose. Hereinafter, these plants comprising lignocellulose may be called lignocellulose as it is or may be called a lignocellulosic raw material.
One of the main applications of a lignocellulosic raw material is a paper product, and fibrous pulp as a raw material of a paper product is prepared from a lignocellulosic raw material such as wood. There are two types of preparation methods of pulp, a method by mechanical crushing and a method by chemical degradation. Lignin is contained in the pulp obtained by the former method, and lignin is removed from the pulp obtained by the latter method.
A paper product produced by using the pulp obtained by the latter chemical degradation has high whiteness and is treated as a high quality paper product. Therefore, in pulp preparation, lignin can be called an undesirable component.
Further, in recent years, various technologies have been studied for utilizing biomass such as wood which is a renewable, organism-derived organic resource unlike fossil fuel resources and the like to obtain bioenergy and a useful organic substance as a measure against energy exhaustion. Development of these technologies has been accelerated with the remarkable progress of biotechnology as a background, and such technology is already entering the stage of practical application.
In this case, of the three lignocellulosic components, two types of polysaccharide raw materials, hemicellulose and cellulose, are considered to be the target for use, because hemicellulose is easily hydrolyzed with an acid and an enzyme, and cellulose is less easily degraded than hemicellulose but dissolves in a specific solvent such as concentrated sulfuric acid or is hydrolyzed by swelling with an alkali followed by treating with a dilute acid. On the other hand, although lignin can be solubilized, for example, with a mixed solution of sodium hydroxide and sodium sulfite or with dioxane or the like, it has undergone significant chemical modification in this process. Therefore, it is difficult to take out lignin in the form as it is present in plants. For this reason, it is considered to be difficult to find a specific target for use of lignin from the viewpoint of biomass utilization.
As a technology for degrading lignin which is difficult to handle, a technology for obtaining a water-soluble organic acid such as gallic acid, oxalic acid or acetic acid by blowing ozone into a lignocellulose suspension to oxidatively degrade the same (refer to Patent Document 1) has recently been disclosed.
Further, a method is disclosed in which, in order to bleach semi-bleached pulp by removing lignin, the pulp is treated by adding 1.2 g of hydrogen peroxide to 100 g of the pulp while irradiating the pulp with microwaves (refer to Patent Document 2). However, since attention is not paid to lignin itself in this case, it is not known what type of product is obtained as a degradation product of lignin.
Patent Document 1: Japanese Patent Laid-Open No. 2006-141244
Patent Document 2: Japanese Patent Laid-Open No. 60-088191